There are a variety of good reasons to hide the grades for an item in the Gradebook at any given time.  For example, you might also wish to keep a set of grades hidden until you have finished grading all the submissions for said item.

Posting Grades

As a rule, the contents of a Gradebook item are not visible to students until they are posted. This means that the column does not need to be hidden, as long as the grades have not yet been posted to students' view–you can treat unposted grades as hidden by default. To post an item's grades, enter the gradebook and click on the item directly.

You will see a list of all the available grades within this item. From here, you can choose to post either individual grades one by one, or post all the graded submissions simultaneously using "Post All Grades". 

Hiding Posted Grades

Posted grades can be hidden by hiding the item they are attached to. To do so, click on the item either within the  Gradebook or the Course Content feed, and select the visibility dropdown in the top right. Select "Hidden from students" to hide the item and its grades from all student view.

I’m trying to hide the Overall Grade and it won’t let me. Why?

Overall Grade Calculations are hidden using a slightly different method, as they are not tied to a specific item. To do so, click on the Overall Grade in the Gradebook. If you see the student Grades, click "Calculation Details". 

on the resulting page, uncheck the box labeled "Show to students", then click "Save" at the bottom of the page. The Overall Grade will no longer be visible to students.